Method and apparatus for correcting errors on a wafer processed by a photolithographic mask

ABSTRACT

The invention relates to a method for correcting at least one error on wafers processed by at least one photolithographic mask, the method comprises: (a) measuring the at least one error on a wafer at a wafer processing site, and (b) modifying the at least one photolithographic mask by introducing at least one arrangement of local persistent modifications in the at least one photolithographic mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/994,556, filed on Oct. 25, 2013, which is a national stageapplication of PCT/EP2011/071654, filed on Dec. 2, 2011, which claimspriority to U.S. Provisional Application 61/424,422, filed on Dec. 17,2010. The contents of the above applications are herein incorporated byreference in their entirety.

TECHNICAL FIELD

The present invention relates to the field of correcting errors on awafer processed by a photolithographic mask.

BACKGROUND

As a result of the shrinking sizes of integrated circuits,photolithographic masks or templates of the nanoimprint lithography haveto project smaller and smaller structures onto a photosensitive layer,i.e. a photo resist dispensed on a wafer. In order to fulfil thisdemand, the exposure wavelength of photolithographic masks has beenshifted from the near ultraviolet across the mean ultraviolet into thefar ultraviolet region of the electromagnetic spectrum. Presently, awave-length of 193 nm is typically used for the exposure of the photoresist on wafers. In order to increase the resolution of thephotolithographic exposure system water is often used as immersionliquid between the projection objective and the wafer. As a consequence,the manufacturing of photolithographic masks with increasing resolutionis becoming more and more complex, and thus more and more expensive aswell. In the future, photolithographic masks will use significantlysmaller wavelengths in the extreme ultraviolet (EUV) wave-length rangeof the electromagnetic spectrum (approximately at 13.5 nm). Doublepatterning lithography is bridging the gap between water-based 193 nmimmersion lithography and EUV lithography.

Photolithographic masks have to fulfil highest demands with respect totransmission homogeneity, planarity, pureness and temperature stability.In order to fabricate photolithographic masks with a reason-able yield,defects or errors of masks have to be corrected at the end of themanufacturing process. Various types of errors of photolithographicmasks and methods for their corrections are described in the USProvisionals U.S. 61/351,056 and U.S. 61/363,352 of a subsidiary of theapplicant, which are hereby incorporated herein in their entirety byreference.

Typically, the basis of photolithographic masks is an ultra-puresubstrate of fused quartz or other low thermal expansion material whichhas on one surface a thin chromium layer or a layer of another non lighttransparent material. The pattern elements of photolithographic masksare generated by a so-called pattern generator based on particle beams,predominantly electrons or a respective laser beam, which write thepattern elements in the absorbing material. In a subsequent etchingprocess, the pattern elements are formed on the substrate of thephotolithographic mask. FIG. 1 schematically illustrates a maskfabrication process. Details of the fabrication process are described inthe fifth section of the specification.

The precise position of the pattern elements on the generated mask ismeasured using a registration metrology tool. When the photolithographicmask exceeds the maximum tolerable positioning error of the patternelements, the mask has to be rewritten. During the re-writing process,it is at first tried to correct the positioning errors of the firstwriting process. However, this works only if the positioning errors aresystematic. The writing time of a critical photolithographic mask may bevery long and may reach a period of up to 20 h. Thus, the repeatedwriting of photolithographic masks is an extremely time-consuming andexpensive process.

In an alternative process, positioning errors of photolithographic maskscan be minimized by the application of a so-called registrationcorrection (RegC) process. As described in the document U.S. 61/361,056,this process uses femtosecond or ultra-short light pulses of a lasersystem to locally change the density of the substrate of aphoto-lithographic mask which results in a shift of the patternplacement on the substrate surface of the photolithographic mask.

In order to protect the structured absorbing layer, a pellicle ismounted on the surface of the photolithographic mask carrying theabsorbing pattern elements. For critical masks, or more precisely ofoverlay-critical masks, the measurement of the position of the patternelements has to be repeated in order to determine the influence of thepellicle on the positioning errors. This process is schematicallyrepresented in FIG. 1.

The generation of an integrated circuit on a wafer requires thesuccessive application of several different photolithographic masks forthe fabrication of the different layers or levels of the component. Theplurality of photolithographic masks necessary for the generation of theintegrated circuit is called a mask set. For an advanced integratedcircuit, the mask set may comprise 20 to 50 different photolithographicmasks. At the end of the mask fabrication process, the complete mask setis transferred from a mask shop to a wafer processing site or to a waferfabrication site.

At the wafer processing site, a projection device successivelyilluminates a wafer by means of the individual photolithographic masksof the mask set in order to transfer the pattern elements of the variousmasks to the respective photo-resistive layer on the wafer. FIG. 2schematically represents this process. By a lithographic process and asubsequent etching process the pattern elements of the photolithographicmask are copied to the wafer forming the respective layer of theintegrated circuit. The overlap accuracy of the differentphotolithographic masks on the wafer is called overlay, and isdetermined by means of overlay targets also copied from thephotolithographic mask to the photo resist layer on the wafer using anoverlay metrology system.

If the overlay error of successive masks exceeds a predeterminedthreshold, the projection device is readjusted, the illumination of thelatest mask is repeated and the overlay error is again measured. Whenthe overlay error still surmounts the overlay budget, the root cause ofthe error has to be analysed and the overlay specification is tightened.The respective mask is sent back to the mask fabrication site or themask shop for rewriting of its pattern elements. As already brieflymentioned, this repair or rewriting process is extremely time-consumingand significantly hampers the wafer processing at the wafer processingsite.

At a wafer processing site, the overlay is presently determined atseveral dedicated targets such as Box in Box, Bar in Bar and AIM(advanced imaging metrology) overlay targets which are arranged at thefour corners of the scribe line of the integrated circuit. The article“Meeting overlay requirements for future technology nodes with in-dieoverlay metrology”, by B. Schulz et al., Proc. SPIE Vol. 6518, 2007,describes that judging the quality of a photolithographic mask by thestandard registration measurement in the scribe line is not at allrepresentative of the placement of the structures in the die. Thissituation can only be improved when the specification of patternplacement errors of photolithographic masks is based on a highersampling plan including representative structures and especiallylocations within the die. The authors of this article report also ofmeasurements on the influence of the pellicle to the overlay error. Theyconclude that this contribution is in the range of 1 nm (3σ value),however, it was too small to be determined with the availablemethodology and the precision levels of the overlay metrology system.

With the extension of the 193 nm ArF (argon fluorine) lithography to the32 nm technology node highest demands are made to the positioning errorsof the photolithographic masks and the overlay accuracy on the wafer.For the 32 nm node, the overlay budget reduces to about 6 nm (3σ value)depending on the device or integrated circuit to be produced. Moreover,the applicant detected that the contribution of the mounting of thepellicle to the positioning error may be significantly larger thanestimated in the above mentioned article. This error may reach adimension of some nanometres, which may take up more than 50% of theoverall overlay budget. This error significantly reduces the yield ofthe overall wafer fabrication process and can therefore not betolerated. Furthermore, the situation is complicated as the influence ofthe pellicle mounting process can only be poorly corrected in advancedue to its insufficient systematics.

Below the 32 nm node, so called double patterning technologies areapplied which require overlay accuracies of below 2.5 nm for someschemes. In double patterning lithography (DPL), the pitch size, whichlimits the patterning resolution, doubles with respect to singlepatterning. The author P. Zimmermann summarizes in the article “Doublepatterning lithography: double the trouble or double the fun?”, SPIENewsroom, Jul. 20, 2009, various double patterning approaches.Presently, three double patterning variants seem to be promising for theapplication in lithography systems:

(a) The litho-etch-litho-etch (LELE) process is schematically shown inFIG. 9. A hard mask (hard mask #1 in FIG. 9) is deposited on the layerwhich is to be patterned (hard mask #2 in FIG. 9). The wafer is exposedwith a first photolithographic mask (first exposure, dark columns isFIG. 9). The hard mask #2 is etched (first etch). Then the wafer isexposed with the second mask (second exposure, dark columns in FIG. 9).Finally, the hard mask #2 is etched and thus forming the combinedpattern of the first and the mask in this layer.

(b) FIG. 10 schematically presents the litho-freeze-litho-etch (LFLE)process. This process works by freezing the developed photo resistpattern of the first exposure, which is symbolized by three columns inFIG. 10. Then a second photo resist layer is added prior to the secondexposure (not shown in FIG. 10). The photo resist pattern of bothphotolithographic masks is then etched in one step after development ofthe photo resist.

(c) The self-aligned double patterning (SADP) process is depicted inFIG. 11. It begins with the deposition of a photo resist layer on thelayer to be etched. In the next step the photo resist layer is exposedand developed. A spacer layer is then deposited over the patterngenerated in the lithography step which covers all pattern elements. Thecovered layer is then selectively etched away leaving two sidewallsalong any ridge. In the next step, the photo resist material is re-movedand the layer is etched wherein the remaining spacers form an etch mask.Finally, the residual spacers are removed.

In double patterning processes, in particular in LELE and LFLEprocesses, critical dimension uniformity (CDU) and overlay errors arecomplex. Furthermore, a double patterning lithography process entanglesCDU and overlay error. In the article “Towards 3 nm overlay and criticaldimension uniformity: an integrated error budget for double patterninglithography” W. A. Arnold discusses the various contributions to theerror budget. He detects that the double patterning lithography processhas profound implications on the CDU as well as on the overlay error. Asalready mentioned this requires overlay errors of about 2.5 nm and CDUvariations of less than 1 nm.

It is therefore one object of the present invention to provide a meth-odand an apparatus for measuring (i.e. “metrology”) and correcting errorson wafers illuminated by a photolithographic mask which at least partlyavoid the problems discussed above.

SUMMARY

According to a first aspect of the invention, a method according topatent claim 1 is provided. In an embodiment, a method for correcting atleast one error on wafers processed by at least one photolithographicmask comprises measuring the at least one error on a wafer at a waferprocessing site, and modifying the at least one the photo-lithographicmask by introducing at least one arrangement of local persistentmodifications in the at least one photolithographic mask.

The defined method measures errors on a wafer generated at theillumination or the exposure of the wafer with a photolithographic maskat the final wafer overlay at the wafer processing site or at the waferfabrication site. Therefore, the inventive method takes all problemsinto account which have influenced the measured overlay errors on thewafer. Since the overlay budget will further shrink with futuretechnology nodes, it will be mandatory to determine the over-all errorfor the mask overlay. It will be more and more difficult to separate thevarious contributions of the overall overlay error and to separatelycorrect them. For example, the inventive principle a priori considersthe impact of the pellicle mounting process to the measured data.Furthermore, the problems of the projection device of thephotolithographic illumination system are also automatically taken intoaccount.

The inventive principle detects errors on the wafer generated by theillumination process of different photolithographic masks. A methodknown as registration correction (RegC) and described in the documentU.S. 61/351,056 allows the calculation of shifts or displacements ofpattern elements, so that the errors detected on the wafer can becorrected by modifying the pattern placement on the respectivephotolithographic mask(s). For this purpose, at the wafer processingsite, femtosecond or ultra-short light pulses of a laser system areapplied in order to modify the density of the mask substrate whichinduces the required pattern placement shifts. By this process, theindividual masks representing different layers of the integrated circuitcan be directly aligned to each other. Hence, the defined method reducesthe alignment errors of different masks resulting in the minimization ofthe overlay error. Thus, the inventive method avoids to a large extentthe involved rewriting of already existing photolithographic masks.

In a further aspect, a method for correcting errors on a wafer processedby a photolithographic mask at a wafer processing site comprisesmeasuring of errors on the wafer, and modifying a pattern placement onthe photolithographic mask by locally applying femtosecond light pulsesof a laser system to the photolithographic mask at the wafer processingsite.

The inventive method can compensate local CD errors and overlay errorsof photolithographic masks, i.e. which are errors not correctable withthe scaling and orthogonality (S/O) corrections of a scanner of theprojection system. Thus, it is well suited to correct local errors ofphotolithographic masks or simply of masks resulting in low over-layerrors at the fabrication of integrated circuits (ICs). Therefore, theinventive method supports the intrOduction of further technology nodeswhich use DPL processes having tight overlay error budgets.

In a further aspect, the at least one error on the wafer comprises atleast one pattern placement error and/or at least one critical dimensionerror and/or at least one overlay error of a plurality ofphotolithographic masks.

The inventive method is not restricted to the correction of overlayerrors originating from pattern placement errors on thephotolithographic mask and/or from alignment problems of the projectionde-vice used in the photolithographic illumination system as well asmask heating, mechanical strains impacting pattern fidelity andadditional contributors to overlay errors on wafer level. It can also beused to correct a variation of the optical transmission resulting in CD(critical dimension) errors across the wafer. Furthermore, the inventivemethod allows simultaneously correcting both types of errors.

In another aspect, the at least one error on the wafer comprises atleast one pellicle mounting error and/or at least one imaging error of aphotolithographic projection exposure system.

The error(s) generated by one or several photolithographic masks aremeasured at the wafer processing site, thus the detected error(s)comprise all the contributions of a real production environment.Be-sides errors of the respective mask(s), it also contains defectportions occurring due to a mounting of a pellicle on a mask and defectswhich are not correctable of the projection exposure system.

According to another aspect, the at least one overlay error comprises atleast one error of at least one of the at least two photolithographicmasks used in a multiple patterning lithography process.

The multiple patterning lithography process may be a double, a triple, aquadruple, etc. patterning lithography process. A multiple patterninglithography requires multiple photolithographic masks for the printingof a single layer on a wafer. Therefore, a multiple patterninglithography processes adds a new contribution to the overlay error. Thisis a placement error of two or several masks which are used to form thepattern for a single layer on the wafer.

In a further aspect, the at least one overlay error comprises at leastone error of at least one of the at least two photolithographic masksused in a double patterning lithography process. In still anotheraspect, the double patterning lithography process comprises alitho-etch-litho-etch (LELE) process, a litho-freeze-litho-etch (LFLE)process, or a self-aligned double patterning (SADP) process.

As already mentioned above DPL processes are presently in the focus aslithography technologies which enable the introduction of furthertechnology modes if the very tight error budgets, in particular withrespect to CDU and overlay errors can be fulfilled. Various features orthe defined method are able to significantly reduce these errors.

According to another aspect, measuring of the at least one errorcomprises measuring of at least one error in the active area of a chip(in-die).

As already mentioned above, the 32 nm technology node and futuretechnology nodes will require the detection of the pattern placement notjust in the scribe line at the four corners of the die, but at a regulargrid on the die itself (in-die). The presented method supports themeasurement of the positioning errors of the pattern elements in-die. Inparticular, an overlay metrology system is now available which enablesthe measurement of positioning errors in the sub-nanometer range, sothat contributions to the overlay error can now be detected which havebeen out of reach up to now.

In still another aspect measuring of the at least one error comprisesusing a scanning electron microscope and/or a scatterometer.

The application of a comprehensive metrology system which may usephotons as well as electrons for detecting errors on a wafer allowon-wafer intra-field overlay metrology with high polynominal orders,i.e. with high resolution and accuracy. It may cover all overlaymetrology use cases. Moreover, the laser source typically used for theintroduction of the arrangement(s) of local persistent modifications ina mask can be performed by separate tools or can be integrated in acomprehensive metrology system. This means that both steps of thecorrection method defined above can be combined in a single tool. Thus,the defined correction method can completely be performed at the waferprocessing site in order to accomplish the final goal to achieve a highquality pattern on the wafer within the wafer fab specification fordimensions, pattern placement and overlay between patterns placed on thewafer. It is also possible to measure the error(s) on a wafer at thewafer processing site and modifying or correcting the mask at the maskshop or elsewhere, or vice versa.

When the process is performed based on wafer data, both mask errors andphotolithography processing errors are handled (i.e. error ofprojection, mask alignment, photo resist processing and additionalcomponents in the photolithography process and equipment modules).

According to another aspect, in-die measuring of the at least one errorcomprises measuring of a shift of at least one two-dimensional and/or atleast one three-dimensional structure on a wafer and/or measuring of anellipticity of at least one two-dimensional and/or at least onethree-dimensional structure with an imaging-based or a model-model-basedmetrology method. In yet a further aspect, in-die measuring of the atleast one error comprises measuring of a shift of at least one contacthole and/or measuring of an ellipticity of at least one contact holewith a scanning electron microscope.

The determination of a shift and of the ellipticity of a plurality ofcon-tact holes or more generally of a two-dimensional and/or ofthree-dimensional structure enables to detect overlay errors and/or CDUerrors of one or both masks in a double patterning lithography processor more generally in a multiple patterning lithography process whichuses a two-dimensional pattern.

In another aspect, in-die measuring of the at least one error on thewafer provides an in-die flag for least one arrangement of localpersistent modifications in the at least one photolithographic mask.

Parallel to an increase in the accuracy of the error detection on thewafer, in-die measurements also provide information where to positionthe arrangement(s) of local persistent modifications in aphoto-lithographic mask which correct or compensate the detectederror(s).

In a further aspect, measuring of the at least one error comprisesmeasuring at a developed photo resist layer on the wafer and/or on thewafer.

The measurement of overlay errors on a wafer can be performed at thedeveloped photo resist on the wafer. Thus, when the detected errors arebelow the predetermined threshold, the processing of the wafer can becontinued. If the detected errors exceed the tolerable level, the lastillumination or exposure step can be repeated by re-moving the photoresist from the wafer and dispensing a new layer of photo resist. Priorto the second illumination, the error of the photo-lithographic mask isalso corrected.

In another aspect, the at least one photolithographic mask comprises apellicle.

In still another aspect, introducing the at least one arrangement oflocal persistent modifications comprises locally applying ultra-shortlight pulses of a laser system onto the at least one photolithographicmask.

According to a further aspect, modifying the photolithographic maskcomprises modifying a pattern placement on and/or an opticaltransmission of the photolithographic mask by introducing at least onearrangement of local persistent modifications. In a further aspect,introducing the at least one arrangement of local persistentmodifications does not introduce a variation of the optical transmissionacross the photolithographic mask. According to still another aspect,introducing the at least one arrangement of local persistentmodifications corrects pattern placement errors and/or opticaltransmission errors of the photolithographic mask.

As already briefly mentioned above, femtosecond or ultra-short lightpulses of a laser system can for example write an arrangement of localdensity variations, called pixels, in a substrate of a photolithographicmask which shift pattern elements on the surface of thephotolithographic mask to a predetermined position. The induced densityvariation of the substrate corrects pattern placement errors on thesurface of the photolithographic mask, and thus minimizing the over-layerror of the mask. On the other hand, an arrangement of pixels can bewritten in the mask substrate which corrects a variation of the opticaltransmission across the photolithographic mask, so that CD errors can becorrected without inducing a shift of the pattern elements on thesurface of the substrate of the photolithographic mask. Moreover, anarrangement of pixels can be defined and written which corrects both,pattern placement errors and optical transmission errors.

In another aspect, introducing the at least one arrangement of localpersistent modifications locally changes a density of a substrate of thephotolithographic mask. Another aspect further comprises the step ofintroducing the at least one arrangement of local persistentmodifications in a centre of the height of the substrate.

The writing of pixels in the centre of the mask substrate avoids abending of the substrate which might introduce image defects resultingin further errors on the wafer illuminated with the respectivephotolithographic mask.

In still a further aspect, the photolithographic mask comprises atransmissive photolithographic mask and/or a reflectivephotolithographic mask and/or a template for the nanoimprintlithography.

The generation of integrated circuits on wafers, which rely on aplurality of any kind of masks, has the problem to align patternelements of different masks. Thus, the inventive method can be used tosolve or at least significantly reduce overlay errors occurring in thesewafer fabrication processes.

In another aspect, the error is at least one overlay error of a majorityof photolithographic masks, the method further comprising the step of:correcting the at least one overlay error by introducing at least onearrangement of local persistent modifications in the at least one of themajority of photolithographic masks. In a further aspect, the majorityof the photolithographic masks are used for a multiple patterninglithography process.

In a beneficial aspect, the error is at least one overlay error of atleast one first photolithographic mask and at least one secondphotolithographic mask, the method further comprising the step of:correcting the at least one overlay error by introducing at least onearrangement of local persistent modifications in the at least one firstphotolithographic mask and/or in the at least second photolithographicmask so that the at least one overlay error is minimized.

According to a further aspect, multiple photolithographic masks are usedin a multiple photolithography process. In another aspect, the at leastone first photolithographic mask and the at least one secondphotolithographic mask are used for a double patterning lithographyprocess.

The overlay error correction method defined in the two precedingparagraphs can be applied to multiple patterning lithography (MPL)processes which use multiple exposures. In particular, it can be appliedto DPL processes which use a double exposure. In a double patterninglithography process often two masks have used to generate the pattern ofa single layer of a wafer. One or both of these masks may contributeerror(s) which manifest in an overlay error of the DPL process.Consequently, the error portions of both masks used in the DPL processare measured, and one or both of these masks are corrected in order tominimize the overlay error of the DPL process.

In a further aspect, correcting the at least one overlay error comprisesintroducing at least one first arrangement of local persistentmodifications in the at least one first photolithographic mask and/orintroducing at least one second arrangement of local persistentmodifications in the at least one second photolithographic mask.According to another aspect, the at least one first arrangement of localpersistent modifications is different from the at least one secondarrangement of local persistent modifications.

It is possible to modify one or both of the masks used for thegeneration of a pattern in a DPL process. It is therefore possible tocompensate a first portion of an overlay error on a first mask and asecond portion of the overlay error on the second mask. Therefore, thearrangements of local persistent modifications in both masks may besimilar or may be different.

In still another aspect, the at least one overlay error comprises atleast one critical dimension uniformity error of the firstphotolithographic mask and/or at least one pattern placement error ofthe second photolithographic mask in a multiple patterning lithographyprocess. In another aspect the multiple patterning lithography processuses a one-dimensional pattern.

According to a further aspect, the at least one overlay error comprisesat least one critical dimension uniformity error of the firstphotolithographic mask and/or at least one pattern placement error ofthe second photolithographic mask in a double patterning lithographyprocess. In another aspect, the double patterning lithography processuses a one-dimensional pattern. In yet a further aspect, correcting theat least one overlay error comprises the step of: introducing at leastone arrangement of local persistent modifications in the at least onefirst photolithographic mask causing a pattern placement modificationand/or introducing at least one arrangement of local persistentmodifications in the at least one second photolithographic mask causinga variation of the optical transmission.

In a further beneficial aspect, measuring the at least one error on thewafer comprises: (a) generating a test mask having a test pattern, (b)printing and etching the test pattern of the test mask on the wafer, (c)printing and etching a photolithographic mask pattern on the testpattern of the wafer, and (d) determining the at least one error as adifference of at least one pattern element of the photolithographic maskand of the at least one respective test pattern element of the testmask.

In a further beneficial aspect, measuring the at least one error on thewafer comprises: (a) generating a test mask having a test pattern,measuring a test mask registration, a critical dimension, and/or adefectivity rate, and documenting the measured data as input for overalloverlay and critical dimension shifts of the entire overlayed structureon the wafer; (b) printing and etching the test pattern of a productpattern of the test mask on the wafer, measuring the printed test maskregistration, the critical dimension, and/or defectivity rate anddocumenting the measured data as input for the overall overlay andcritical dimension shifts of an entire overlayed structure on the wafer;(c) printing and etching a photolithographic mask pattern on the testpattern or the product pattern of the wafer, wherein the test maskpattern is measured prior to the product mask pattern or vice versa, and(d) determining the at least one error as a difference of at least onepattern element of the photolithographic mask based on the documentedmeasured data of registration shifts and/or critical dimension shifts ofeach printed layer to accomplish the entire overlayed structures on thewafer; and (e) of the at least one respective test pattern element ofthe test mask.

In still a further aspect, printing and etching of the test mask andprinting and etching of the photolithographic mask comprises a singlepatterning lithography process or a multiple patterning lithographyprocess.

In still a further aspect, printing and etching of the test mask andprinting and etching of the photolithographic mask comprises a doublepatterning lithography process.

The test pattern of a test mask provides a reference pattern which isinvestigated in detail to have the predetermined shape and is arrangedat the predetermined position. Therefore, the major portion of theoverlay error of a test mask and a mask used in a wafer fabricationprocess in a single of a MPL process comes from the production mask.This means that the pattern of the photolithographic test mask has tofit to the pattern of the photolithographic production masks.

Still another aspect further comprises the step of determining anoverlay error from a shift of a plurality of test pattern elements ofthe test mask relative to pattern elements of the photolithographic maskused in the single patterning lithography or multiple patterninglithography process.

Another aspect further comprises the step of determining an overlayerror from a shift of a plurality of test pattern elements of the testmask relative to pattern elements of the photolithographic mask used inthe double patterning lithography process. Still a further aspectcomprises correcting the overlay error by using a scaling andorthogonality correction functionality or a scanner or of a stepper.

Beside the detection of local errors of a mask, the test pattern of thetest mask can also be used to detect an overlay error of thephotolithographic mask used with the test mask in a DPL process.

According to another aspect, the test pattern comprises in-die testfeatures.

As already mentioned, this feature secures a high resolution at theerror determination. Moreover, this feature allows a precisedetermination of the position at which the correcting pixels have to beintroduced in the substrate of the photolithographic mask.

A further aspect comprises the test pattern with dense pattern elements,medium dense pattern elements, and isolated pattern elements.

Due to proximity effects in the lithography and due to etch loadingeffects the widths of dense lines and isolated lines having theidentical width on the mask are different on a wafer. Having dense linesand isolated lines available on the test pattern allows controlling theeffect of a mask on both, pattern elements as well as on assist featuresas optical proximity correction (OPC). The test mask pattern should takeconsiderations into account related to pattern dimensions, placement,overlay and fidelity in order to generate high quality metrology datathat will later be used as an input for the correction of the aboveconsiderations.

In yet another beneficial aspect, the test pattern of the test maskcomprises pattern elements adapted for determining at least one overlayerror of at least one overlay critical photolithographic mask. Stillanother aspect further comprises the step of measuring the overlay errorof each overlay critical photolithographic mask.

By measuring the error(s) of the overlay critical masks the error budgetof the overlay error of the IC fabrication process can be con-trolled.By compensating error(s) of overlay critical masks, tight error budgetsof MPL or DPL processes can be met.

Still a further aspect comprises the step of introducing at least onearrangement of local persistent modifications in a photolithographicmask used in a self-aligned double patterning process. Another aspectfurther comprises the step of introducing the at least one arrangementof local persistent modifications in a sacrificial layer on the waferused in a self-aligned double patterning process. In a further aspect,the at least one arrangement of local persistent modifications correctsa critical dimension uniformity error of a sacrificial layer arranged onthe wafer.

CDU error(s) in the sacrificial layer of a SADP process causesover-lay-like error(s) in the final etched pattern. This effect is knownas “pitch walking”. By in-die measurements of the spacer elementsforming the etch mask, a variation of the lines and of the spaces of theSADP processes can be compensated by locally modifying the opticaltransmission of a mask so that the pattern generated with a modified orcorrected mask counteracts the lines and the spaces variation.

According to a further aspect, an apparatus for correcting at least oneerror on wafers processed by a photolithographic mask comprises (a) atleast one metrology system located in a wafer processing site and/or ina mask shop and adapted to measure the at least one error on a wafer,(b) at least one computing means adapted to calculate parameters for atleast one error correction means based on the at least one measurederror, and (c) the at least one error correction means is adapted tointroduce at least one arrangement of local persistent modifications inthe photolithographic mask by applying ultra-short light pulses.

In another aspect, an apparatus for correcting errors on a waferprocessed by a photolithographic mask at a wafer processing sitecomprises (a) at least one overlay metrology system adapted formeasuring of errors on the wafer, (b) at least one computing meansadapted for calculating an arrangement of femtosecond light pulses forthe photolithographic mask from measured error data, and (c) at leastone laser system adapted for modifying a pattern placement on thephotolithographic mask by applying the arrangement of femtosecond lightpulses on the photolithographic mask.

In a further aspect, the apparatus is adapted to perform a methodaccording to any of the aspects defined above.

According to still a further aspect, the at least one error correctionmeans comprises at least one laser system. In another aspect, the atleast one laser system is adapted to generate ultra-short laser pulses,in particular femtosecond laser pulses.

Finally, in another aspect, the at least one metrology system comprisesan ultrahigh-precision stage, at least one laser source and at least onecharge-coupled device camera operating in the ultraviolet wave-lengthrange and/or a scanning electron microscope and/or a scatterometerand/or an image-based or a model-based metrology system.

DESCRIPTION OF DRAWINGS

In order to better understand the present invention and to appreciateits practical applications, the following Figures are provided andreferenced hereafter. It should be noted that the Figures are given asexamples only and in no way limit the scope of the invention.

FIG. 1 schematically represents a flow chart of a fabrication process ofa set of photolithographic masks according to the prior art;

FIG. 2 schematically shows a flow chart of a use case of a set ofphotolithographic masks at a wafer processing site according to theprior art;

FIG. 3 schematically represents a block diagram of some of the majorcomponents of an apparatus used for measuring overlay errors on a wafer;

FIG. 4 schematically shows a block diagram of an apparatus used forcorrecting overlay errors in the substrate of a photolithographic mask;

FIG. 5 schematically represents a flow chart of a fabrication process ofa set of photolithographic masks according to an embodiment of theinventive method;

FIG. 6 schematically shows a flow chart of a use case of a set ofphotolithographic masks at a wafer processing site according to anembodiment of the inventive method;

FIG. 7 schematically represents a displacement vector map measured atthe scribe lines of dies;

FIG. 8 schematically illustrates a displacement vector map measured atnodes of a regular grid, i.e. at the scribe lines of dies and in-die;

FIG. 9 schematically presents process steps of a litho-etch-litho-etch(LELE) process;

FIG. 10 schematically shows process steps of a litho-freeze-litho-etch(LFLE) process;

FIG. 11 schematically depicts process steps of a self-aligned doublepatterning (SADP) process;

FIG. 12 schematically illustrates critical dimension uniformity andoverlay errors of a cut-out of a one-dimensional pattern generated witha LELE or LFLE process;

FIG. 13 schematically shows a cut-out of a two-dimensional patterngenerated with a LELE or LFLE process;

FIG. 14 repeats FIG. 13, wherein the horizontal pattern has an overlayerror;

FIG. 15 illustrates FIG. 13, wherein the vertical pattern has a criticaldimension uniformity error;

FIG. 16 represents FIG. 13, wherein the horizontal pattern has anoverlay error and the vertical pattern has a critical dimensionuniformity error;

FIG. 17 schematically shows enlarged the contact holes of FIG. 15 whichare elliptically deformed due to the critical uniformity error;

FIG. 18 schematically presents process steps and their variation of aself-aligned double patterning (SADP) process;

FIG. 19 schematically depicts a test pattern of a test mask;

FIG. 20A schematically shows a photo resist on a wafer after to theexecution of second litho-etch steps in a LELE process havingessentially a plane surface due to an introduction of a hard mask filmwith high optical contrast to the surface and photoresist layer toenable high resolution metrology in high accuracy, sensitivity andprecision;

FIG. 20B schematically shows a photo resist on a wafer after to theexecution of second litho-etch steps in a LELE process having apronounced surface topography;

FIG. 21 schematically presents a LELE process of a test mask pattern andof a mask pattern having vertical line space features;

FIG. 22 schematically depicts a LELE process of a test mask pattern andof contact holes of a mask used in the DPL process;

FIG. 23 schematically shows a LELE process of a test mask pattern and ofa mask pattern having horizontal line space features;

FIG. 24 schematically illustrates a superposition of the test pattern ofthe test mask of FIG. 19 and of the mask patterns of FIGS. 21 to 23;

FIG. 25 schematically depicts the superposition of the test pattern ofthe test mask of FIG. 19 and of the mask patterns of FIGS. 21 to 23,wherein the errors of the mask patterns of FIGS. 21 to 23 are corrected;

FIG. 26 schematically presents in-field targets for the determination ofan averaged in-field overlay error;

FIG. 27 schematically shows a wafer covered with fields, wherein eachfield has the overlay targets of FIG. 26;

FIG. 28 schematically reproduces an overlay error map across a wafer;

FIG. 29 schematically illustrates an in-field error map resulting froman averaging of the in-field error distributions across the wafer; and

FIG. 30 schematically depicts the averaged in-field error map of FIG. 29after the systematic errors are corrected by introducing respectivearrangements of local persistent modifications in the substrate of thephotolithographic mask used for the printing of the wafer of FIG. 28.

DETAILED DESCRIPTION

In the following, the present invention will be more fully describedhereinafter with reference to the accompanying Figures, in whichexemplary embodiments of the invention are illustrated. However, thepresent invention may be embodied in different forms and should not beconstrued as limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andwill convey the scope of the invention to persons skilled in the art.

This first section describes an embodiment of the inventive method. Toillustrate the inventive principle, differences are highlighted in thefabrication of a set of photolithographic masks with respect to thefabrication according the prior art. Further, these differences are alsoexemplified for a use case of a mask set. The inventive method isexplained for the utilization of transmissive photolithographic masks.However, the person skilled in the art will appreciate that this is justan example and that the above defined method can also be applied toreflective photolithographic masks. Moreover, the inventive method isalso well suited to correct overlay errors of templates for thenanoimprint lithography at a wafer processing site or at a waferfabrication site.

In the following, the term integrated circuit (IC) is used for alldevices fabricated on semiconducting wafers as for example memory orlogic components, MEMS (micro-electromechanical systems) includingsensors, detectors and displays and PICs (photonic integrated circuits)including lasers and photodiodes.

The inventive method corrects errors detected on a wafer by applyingfemtosecond light pulses of a laser system to a photolithographic maskwith which a wafer was illuminated. For the correction feature of thepresent invention, the specification refers to the US provisional withthe No. 61/363,352. This document describes in detail how patternplacement errors can be corrected by the writing of a respectivearrangement of pixels in the substrate of photolithographic masks. Someof the problems of photolithographic masks and of templates for thenanoimprint lithography are also briefly discussed in the abovementioned document.

FIG. 1 outlines the fabrication process 100 for a mask set according tothe prior art. The process begins at 105 with the writing of the pattern115 of the first mask 110. Then, also at step 115, the excessive partsof the absorber layer are removed from the mask substrate, for exampleby etching. In the next step 120, the photolithographic mask is measuredin order to determine the positioning errors of the mask pattern. Atdecision block 125, it is decided whether the positioning errors fulfila predetermined specification. If this is correct, the pellicle ismounted on the mask in step 135. It is then decided at decision block145 whether the mask is an overlay critical mask. The fabricationprocess is complete when mask m is not an overlay critical mask. If maskm is overlay critical (decision block 145), the mask is again measuredat step 155 to check the effect of the pellicle mounting on thepositioning errors. When it is determined at decision block 165 thatmask m does not fulfil the specification, at step 160, the pellicle isremoved and the pattern is rewritten in a new mask.

When it is determined at decision block 125 that the positioning errorsof mask m do not fulfil the predetermined specification, at step 130,the pattern placement errors are corrected by using a so called RegC(Registration Correction) process, which is described in detail in theU.S. 61/351,056 and U.S. 61/363,352. If the positioning errors cannot bereduced by a RegC process so that mask m meets the specification, thepattern is written on a new mask and the process proceeds to step 120where the mask is measured.

The process of FIG. 1 is repeated until all M masks of the mask set arefabricated (decision blocks 150 and 175). The complete mask set is thensupplied to the wafer processing site at block 180.

FIG. 2 briefly illustrates some steps of a mask illumination process forthe fabrication of ICs at a wafer processing site. The process 200begins at 205 with the illumination or the exposure of a wafer (step215) with the first mask (step 210) of the mask set. The photo resist onthe wafer is developed and the wafer is processed, for example byperforming of an etching process. Then a second (generally m^(th)) layerof photo resist is arranged on the wafer (not illustrated in FIG. 2). Atstep 220, the wafer is illuminated with the second, or generally withthe (m+1)^(th) mask. The overlay errors between the first and the secondmask, or in general between the m^(th) and the (m+1)^(th) mask on thewafer are measured at step 230. If it is decided at decision block 240that the overlay error is below the predetermined overlay budget, it isat decision block 265 checked whether the IC to be fabricated willoperate according to its specification. If this is correct, the processproceeds via decision block 295 and step 225 to block 220 where thewafer is illuminated with the next (third or (m+2)^(th)) mask. When itis determined at decision block that m reaches M which is the number ofmasks in the respective mask set, the process ends at block 270.

If it is decided at decision block 265 that the IC will not operateaccording to its specification, the root cause of the problem isanalyzed at block 280 and the positioning specification for mask m+1 istightened. In step 290, a new mask m+1 is then written at the maskprocessing site. The process ends at block 270. Then the new mask m+1 issupplied from the mask fabrication site to the wafer processing site,the process begins again at block 205 of FIG. 2.

When it is decided at decision block 240 that the overlay error does notmeet the specification, the projection device of the illumination systemis readjusted at block 245. Then, at step 260, the overlay errormeasurement is repeated. If it is determined at decision block 275 thatthe overlay error does still not fulfil the predetermined error budget,the process proceeds to step 280 and the cause of the problem isanalyzed. In case the overlay error meets the specification, it is atdecision block 285 determined whether the IC to be fabricated willoperate according to its specification. If this is true, the processproceeds via decision block 295 across step 225 to block 220 where thewafer is illuminated with the next (third or (m+2)^(th)) mask of themask set. Alternatively, when the illuminated mask is the last mask ofthe mask set (m=M), the process ends at block 270.

FIG. 3 shows a functional sketch of a registration metrology tool 300with which pattern placement and overlay errors can be measured. Aphotolithographic mask 310 is supported by the high precision stage 320.The stage 320 is actively controlled in all six degrees of freedom andis the only moving part in the metrology system. As a light source anexcimer laser 330 is used emitting light in the DUV (deep ultraviolet)wavelength range, at approximately 193 nm. This means that theinspection and the illumination of the photolithographic mask 310 occurat the same wavelength, as most masks are presently illuminated with a193 nm light source. Hence, the registration and/or overlay metrologysystem 300 takes the effect of material properties properly intoaccount.

The imaging objective 340 has a numerical aperture (NA) of 0.6, but canbe extended to a higher NA in order to gain even more resolving power.The short wavelength of the laser system 330 significantly improves theresolution, and at the same time permits a moderate NA, which isbeneficial for the CD (critical dimension) metrology and enables apellicle compatible free working distance of about 7.5 mm. The imagingobjective 340 is firmly fixed to the optical tower and is unmovable.Focusing of the laser beam onto the photolithographic mask 310 is doneby a stage movement in z direction.

A CCD (charge-coupled device) camera 350 is used as a detector devicewhich measures the light reflected from the photolithographic mask 310.The CCD camera 350 sends its signal to the signal processing unit 355which calculates an image of the signal detected by the CCD camera 350.

A computer system 360 can display the image calculated by the signalprocessing unit 355 and may store the measured data. Further, thecomputer system 360 may contain algorithms, realized in hardware,software or both, which allow to extract control signals from theexperimental data. The control signals may control the writing of anarrangement of pixels in the substrate of the photolithographic mask 310by a second laser system in order to correct the pattern placementerrors of photolithographic mask 310 (cf. FIG. 4 below). Further, thecomputer system 360 may control the laser source 330 and/or thehigh-precision stage 320 and/or the objective 340 and/or the CCD camera350 and/or the AF system 370.

The surface of the photolithographic mask 310 may be slightly tilted,and in addition the bending of the mask 310 under its own weight leadsto a variation of the best focal position. Therefore, the registrationmetrology tool 300 has an autofocus (AF) system 370 based on a tiltedgrating (not shown in FIG. 3) which supports the measurement process.The tilted mirrors 390 and the partially transmitting mirrors 395 directthe laser beam into the imaging objective 340.

Furthermore, the registration metrology tool 300 comprises an auxiliaryoptical system 380 for a coarse alignment of the pattern placementelements on the photolithographic mask 310.

FIG. 4 depicts a schematic block diagram of an apparatus 400 which canbe used to correct errors on wafers by modifying the substrate of aphotolithographic mask. Further, the apparatus 400 is also able tocorrect errors of templates used in the nanoimprint lithography. Theapparatus 400 comprises a chuck 420 which may be movable in threedimensions. The photolithographic mask 420 or a template for thenanoimprint technique may be fixed to the chuck 420 by using varioustechniques as for example clamping.

The apparatus 400 includes a pulse laser source 430 which produces abeam or a light beam 435 of pulses or light pulses. The laser source 430generates light pulses of variable duration. The adjustable range ofseveral import parameters of the laser source 430 is summarized in thefollowing table. Table 1 represents an overview of laser beam parametersof a frequency-doubled Nd-YAG laser system which can be used in anembodiment of the inventive method.

TABLE 1 Numerical values of selected laser beam parameters for a Nd-YAGlaser system Overview Numerical Parameter value Unit Pulse energy 0.05-5 μJ Pulse length  0.05-100 ps Repetition rate    1-10 000 kHzPulse density 1 000-10 000000 mm-2 NA (numerical  0.1-0.9 aperture)Wavelength 532 nm

In an alternative embodiment of the laser system the light pulses may begenerated by a Ti: Sapphire laser operating at a wavelength of 800 nm.However, the correction of pattern placement errors is not limited tothese laser types, principally all laser types may be used having aphoton energy which is smaller than the band gap to the substrate of thephotolithographic mask 410 and which are able to generate pulses withdurations in the femtosecond range.

The steering mirror 490 directs the pulsed laser beam 435 into thefocusing objective 440. The objective 440 focuses the pulsed laser beam435 onto the photolithographic mask 410. The NA (numerical aperture) ofthe applied objectives depends on the predetermined spot size of thefocal point and the position of the focal point within thephotolithographic mask 410 or of the template. As indicated in table 1,the NA of the objective 440 may be up to 0.9 which results in a focalpoint spot diameter of essentially 1 μm and a maximum intensity ofessentially 10²⁰ W/cm².

The apparatus 400 also includes a controller 480 and a computer system460 which manage the translations of the two-axis positioning stage ofthe sample holder 420 in the plane perpendicular to the laser beam (xand y directions). The controller 480 and the computer system 460 alsocontrol the translation of the objective 440 perpendicular to the planeof the chuck 420 (z direction) via the one-axis positioning stage 450 towhich the objective 440 is fixed. It should be noted that in otherembodiments of the apparatus 400 the chuck 420 may be equipped with athree-axis positioning system in order to move the photolithographicmask 410 to the target location and the objective 440 may be fixed, orthe chuck 420 may be fixed and the objective 440 may be moveable inthree dimensions. Although not economical, it is also conceivable toequip both the objective 440 and the chuck 420 with three-axispositioning systems. It should be noted that manual positioning stagescan also be used for the movement of the mask 410 to the target locationof the pulsed laser beam 435 in x, y and z directions and/or theobjective 440 may have manual positioning stages for a movement in threedimensions.

The computer system 460 may be a microprocessor, a general purposeprocessor, a special purpose processor, a CPU (central processing unit),a GPU (graphic processing unit), or the like. It may be arranged in thecontroller 480, or may be a separate unit such as a PC (personalcomputer), a workstation, a mainframe, etc. The computer 460 may furthercomprise I/O (input/output) units like a keyboard, a touchpad, a mouse,a video/graphic display, a printer, etc. In addition, the computersystem 460 may also comprise a volatile and/or a non-volatile memory.The computer system 460 may be realized in hardware, software, firmware,or any combination thereof. Moreover, the computer 460 may control thelaser source 430 (not indicated in FIG. 4). The computer systems 360 ofFIG. 3 and 460 of FIG. 4 may be connected to exchange data. Moreover,the computer systems 360 and 460 may be combined in a single computersystem.

Further, the apparatus 400 may also provide a viewing system including aCCD (charge-coupled device) camera 465 which receives light from anillumination source arranged in the chuck 420 via the dichroic mirror445. The viewing system facilitates navigation of the photolithographicmask 410 to the target position. Further, the viewing system may also beused to observe the formation of a modified area on the substratematerial of the mask 410 by the pulsed laser beam 435 of the lightsource 430.

FIG. 5 schematically depicts an example of a fabrication process ofphotolithographic masks of a mask set according to the inventive method.As already briefly explained in the second section of thisspecification, a pattern of absorbing elements is written on anabsorbing layer on the substrate of a photolithographic mask with apattern generator. In a subsequent etching process, the absorbingpattern elements are formed from the absorbing material (box 515). Amaterial often used for the absorbing layer on photolithographic masksis chromium. Tungsten can be used as another absorber material on thesurface of mask substrates. The usage of the inventive method is notrestricted to these materials; rather any absorber material can be used.

The positions of the generated absorbing pattern elements are measuredwith the registration metrology system of FIG. 3 in order to determinewhether the pattern writing process was successful, i.e. the patternelements have their predetermined size and form and are at the desiredpositions (box 520). If the determined positioning errors exceed apredetermined level (decision box 525), the positions of the patternelements are modified by writing of an arrangement of pixels into thesubstrate of the photolithographic mask using the laser source 430 ofthe apparatus 400 of FIG. 4 (box 530 of FIG. 5). The arrangement ofpixels locally changes the density of the mask substrate and thus shiftsthe pattern elements on the mask surface to the predetermined positions.Then, it is measured whether the repair of the mask was successful (box520). If the measured positioning error is now below the predeterminedthreshold (decision box 525), the pellicle is mounted on the surface ofthe photolithographic mask carrying the absorbing pattern elements inorder to protect them from being damaged. When all masks of the mask setare processed according to this scheme, the fabricated mask set is readyfor the delivery to the wafer processing site.

As can be seen from FIG. 5, an embodiment of the mask fabricationprocess according to the inventive method largely avoids rewriting ofphotolithographic masks. If the positioning errors of a mask exceed thetolerable level, the respective mask is corrected by using the RegCprocess.

The mask fabrication process of FIG. 5 abandons of the measurement ofpositioning errors for overlay-critical photolithographic masks aftermounting of the pellicle. This is the decisive distinction between themask fabrication processes according to FIGS. 1 and 5.

FIG. 6 schematically illustrates a use case of a set ofphotolithographic masks at the wafer processing site according to anembodiment of the inventive method. The process begins by illuminatingof a wafer with a first mask using the projection device of thephotolithographic illumination system (box 615). The first mask may bethe first mask of a mask set, or in the general case, it may be anymask, but the last one of the mask set. The photo resist is developed,and the wafer is processed to generate a first layer, or generally anm^(th) layer, respectively, of an integrated circuit. Then a new photoresist layer is dispensed on the wafer (not shown in FIG. 6).

In the next step (box 620), similar to the first photolithographic mask,a second photolithographic mask is aligned with respect to alignmentmarks on the wafer. Then the second mask, or generally the (m+1)^(th)mask, is illuminated similar to the first mask in order to transfer thestructure elements for the second layer, or general (m+1)^(th), layer ofthe integrated circuit from the photolithographic mask to the wafer. Thephoto resist is then developed.

The photolithographic masks have overlay targets which are used todetermine the overlay of the second mask with respect to the firstphotolithographic mask. The standard overlay targets are BiB(box-in-box) targets, which allow the detection of shifts or ofdisplacements of the second mask relative to the first mask. Since theBiB targets have a rather coarse structure, they are now more and morereplaced by AIM (advanced imaging metrology) and micro AIM overlaytargets.

Up to now, the overlay targets are positioned in the scribe lines of theintegrated circuits. FIG. 7 schematically presents a displacement vectormap or a displacement vector field measured at overlay targets arrangedin scribe lines of integrated circuits. The arrow tips of the individualvectors of the displacement vector field indicate the directions of thedisplacement of the respective positions of the second mask with respectto the first photolithographic mask. The lengths of the vectors denotethe magnitude of the shift of the respective positions of the secondmask relative to the first mask. It can be seen from FIG. 7 that therestriction of the placement of the overlay targets to the scribe linesresults in an irregular distribution of the overlay measuring pointsacross the wafer.

With shrinking sizes of the structures of integrated circuits and, onthe other hand, increasing sizes of integrated circuits, it will nolonger be sufficient to determine the overlay at the scribe line, butnot on the die on the integrated circuit itself (in-die measurement).FIG. 8 schematically shows a grid of regular nodes of overlay measuringpoints where the nodes are arranged in the scribe lines as well as onthe die area of the integrated circuit itself. This dense grid ofoverlay measuring points allows the determination of the overlay errorwith a high spatial resolution. A dense grid of overlay measuring pointsis a prerequisite for a beneficial application of the inventive method.

Now back to FIG. 6, the overlay error of the second ((m+1)^(th)) maskrelative to the first (m^(th)) mask can be measured by using the overlaymetrology system 300 of FIG. 3 in order to determine a displacementvector field similar to FIG. 8. When the measured overlay error is belowa predetermined threshold and the fabricated integrated circuit is atthe end working properly, the first and the second masks can be used forthe fabrication of the desired integrated circuit.

If the measured overlay error does not fulfil the predeterminedspecification, the projection device is readjusted in order to reducethe overlay error (box 645). After removing the photo resist from thewafer, a new layer of photo resist material is dispensed on the wafer(not shown in FIG. 6). Then, the overlay measurement is repeated (box660). When the measured overlay error fulfils the requirement withrespect to the predetermined overlay error, the first and the secondmasks are ready for being used for the production of the respectiveintegrated circuit.

In case the overlay error is still too high, the overlay errors areanalyzed based on measured displacement vectors as indicated in FIG. 8in order to determine an arrangement of pixels for the secondphotolithographic mask (box 680). The writing of the arrangement ofpixels with the laser source 435 of the apparatus 400 of FIG. 4 in thesubstrate of the second photolithographic mask shifts the patternelements on the surface of the second photolithographic (box 690).

The writing of pixels can be limited to the active area of thephotolithographic mask. The correction of pattern placement errors inthe active area of the photolithographic masks is very effective, sincethe correcting pixels can be placed close to the error positions. On theother hand, when the writing of pixels is not restricted to the activearea, the flexibility of the error correction process is enhanced. Ifthe writing of the pixels can be limited to the non-active area, theintroduction of new errors in the active area of the substrate of thephotolithographic mask by the pixel writing process can be avoided.Since the distance between the pattern placement error and thecorrecting pixels may be large, the effectiveness of the correctionprocess may be lower. This may partly be compensated as the writing ofpixels does not have to consider a variation of the opticaltransmission.

After correcting of the second photolithographic mask, the wafer isprepared for a second illumination or exposure with the corrected secondmask as described above. At the second illumination of the correctedsecond mask, its overlay error with respect to the firstphotolithographic mask is significantly reduced, so that the maskcombination fulfils the predetermined overlay error.

In contrast to FIG. 2, the use case of a set of photolithographic masksaccording to FIG. 6 eliminates the necessity to send defectivephotolithographic masks from the wafer processing site to the maskfabrication site. Moreover, since the same metrology tool is used forthe measurement of the overlay error and of the correctedphotolithographic mask, tool related impacts on the measured data areavoided.

The following second section describes examples of the application ofthe inventive principle to double patterning lithography (DPL)processes. As already mentioned, in DPL the pitch size, which typicallylimits the patterning resolution, can be doubled for a pattern on awafer compared to a single illumination or exposure. The inventiveprinciple is in the following described in the context of DPL processes.However, it is recognized that the inventive principle can also beapplied to lithography processes which use more than twophotolithographic masks or more than two patterning processes for thegeneration of a pattern of a single layer on a wafer.

A first example describes the critical dimension uniformity (CDU) andoverlay process control in litho-etch-litho-etch (LELE) andlitho-freeze-litho-etch (LFLE) DPL processes. FIG. 12 schematicallydepicts a cut of a cut-out of one-dimensional pattern generated with aLELE of LFLE process. As briefly discussed at the description of FIGS. 9and 10, the first lithography and etch step generates the lines L1(indicated in dark grey in FIG. 12). The second lines L2 (light greyrectangles in FIG. 12) are fabricated a in second litho-etch process.The combined DPL pattern results in lines L1 and L2 which are separatedby a first space S1 between lines L1 and L2 and by a second space S2between lines L2 and L1.

The upper row of lines and stripes of FIG. 12 shows a perfect form ofthe lines L1 and L2. Moreover, lines L1 and L2 are at theirpredetermined positions, and thus also automatically generating spacesS1 and S2 having an identical stripe width.

The middle part of FIG. 12 illustrates a situation in which the lines L2have a perfect form and are located exactly at their predeterminedposition. On the other hand, the lines L1 also have their predeterminedposition but their width decreases in the example of FIG. 12 from leftto right. The decreasing width of the lines L1 leads to a correspondingincrease in the widths of the spaces S1 and S2. The decreasing width ofthe lines L1 results in CDU errors of lines L1 as well as of the spacesS1 and S2. These CDU errors can be corrected by introducing anarrangement of local persistent modifications in mask 1 which generatesthe pattern of lines L1 of the upper part of FIG. 12.

The CDU problem can be compensated by varying the optical transmissionof mask 1 in the portion not having the CDU variation a in the range ofup to some percent, i.e. by artificially decreasing the opticaltransmission of the defect free portions of mask 1. The portions of mask1 having a local CDU variation are compensated by locally varying theportions of mask 1 with respect to the portion having the maximum CDUerror (which is the left part or the middle row of FIG. 12). Arespective increase the exposure dose of mask 1 by the projection systemof the photolithographic exposure system corrects the CDU variation ofmask 1 to a large extent.

The lower part of FIG. 12 presents perfect forms or shapes of lines L1and also lines L2. However, in the cut-out of FIG. 12 the lines L2 areshifted with respect to their predetermined position which is indicatedby the vertical dash dotted lines. The shift of lines L2 leads a smallerwidth of spaces S1 and to a broader width of spaces S2 compared thepredetermined space widths illustrated in the upper part of FIG. 12.This results in CDU errors of spaces S1 and S2.

The CDU errors of spaces S1 and S2 are corrected by introducing at leastone arrangement of local persistent modifications in mask 2 of the DPLprocess which compensates the local pattern placement error of the linesL2 of mask 2 in the cut-out of the line space pattern presented in FIG.12.

If a line shape pattern shows the combined errors of the middle and thelower part of FIG. 12, these errors can also be corrected by introducingat least one respective arrangement local persistent modifications inmask 1 as well as in mask 2.

The CDU variation of mask 1 in the middle part of FIG. 2 and the localshift problem or the local overlay error of the lines L2 of mask 2manifest in an overlay error in the DPL process. This means that a DPLprocess entangles local errors of the individual masks forming thecombined pattern of a layer on a wafer to overlay errors of the patternof the single layer. It is therefore mandatory to correct local errorsof the individual masks of a DPL process in order to meet the tightoverlay budget of technology nodes smaller than 32 nm.

As already mentioned, a prerequisite for the error correction or errorcompensation in DPL processes or generally in multiple patterninglithography (MPUL) processes is a determination of the local errors withhigh resolution. In FIG. 3 a high resolution metrology tool has beenpresented which uses photons for the defect detection. In addition, ascanning electron microscope (SEM) as well as a scatterometry, aproliferometry, an atomic force microscope (AFM) or other metrology tooltypes can also be applied in order to detect local defects ofphotolithographic masks which manifest in printing errors on a wafer.Electrons in an electron beam can be focussed to a small spot, thus anSEM can resolve structures down to the nanometre range.

There are additional metrology technologies available which have aresolution in a sub-nanometer range. Most high resolution imagingtechnologies have a trade-off due to a high numerical aperture (NA), andare hence very limited in the depth of focus (DOF) and have limitedimaging capabilities closed to the wafer surface. Consequently, suchtools may have difficulties to perform the imaging of the top layer andof previous layer(s) that are expected to be aligned. The test maskmethod (serving both single and multiple patterning) is solving thisissue in multiple surfaced layers while the other methods are limited tomultiple patterning, where all layers of interest for the specificprocess step are placed in the same topographical surface.

As already mentioned, MPL processes comprise double, triple, quadruple,quintuple, etc. patterning lithography processes. Presently, DPLprocesses are preferred.

In order to provide the high resolution required for the fabrication ofpatterns of technology nodes smaller than 32 nm, in-die measurements arerequired on the wafer. The metrology tools discussed above can beutilized for this purpose. Furthermore, the high-resolution errordetermination by in-die measurements simultaneously provides anindication or sets a local flag at the position of the error correctionby introducing at least one arrangement of local persistentmodifications in the substrate of the respective photolithographic mask.

At the moment, the pitch size of a single exposure lithography processis limited to about 60 nm for lithography processes having a highuniform pattern. However, the fabrication of state of the art ICsrequires a pitch size of less than 40 nm. FIG. 13 shows atwo-dimensional (2D) pattern 1300 generated in a DPL process which canachieve pitch sizes of less than 40 nm in two vertical directions. Thehorizontal lines 1310 are a regular arrangement of lines separated byregular spaces 1320 which are generated by a first mask. The verticallines 1330 are a regular arrangement of lines separated by regularspaces 1340 which are generated by a second mask in a LELE or an LFLEprocess. The circles 1350 symbolize schematically the designed shape andthe predetermined location of an arbitrary pattern element which is usedas a reference pattern element. Reference pattern elements 1355 having aform similar to the element 1350 are for example a contact hole (CH), avia, a memory capacitor, micro-electro mechanical system (MEMS)elements, or any other 2D and/or three-dimensional (3D) electronicdevice.

As will be seen in the following Figures, any deviation from the CDUspecification will cause a distortion of the printed 2D pattern.Moreover, any local shift or displacement of the lines 1310 or 1330 ofone of the first or the second mask also results in a distortion of theprinted 2D pattern.

The diagram 1400 of FIG. 14 schematically illustrates the 2D pattern1300 of FIG. 13, wherein the first mask which prints the horizontallines 1310 has a local pattern placement problem or local overlay errorwith respect to a predetermined line position of the first mask. Thedisplacement problem of FIG. 14 leads to the printing of lines 1415instead of lines 1310. The displacement caused by a shift of lines 1415is represented in the cut-out 1400 of FIG. 14 by a shift of thereference pattern elements 1350 of FIG. 13 in the vertical direction toa new position 1455 in FIG. 14. In essence, the overlay of printed(horizontal) lines 1415 will cause an overlay issue of the contact holesor any other 2D features, and the same goes for any shift in thevertical lines 1330.

The local shift of the reference pattern elements 1455 of FIG. 14 can becompensated by introducing local arrangement(s) of local persistentmodifications in the first mask generating the horizontal lines 1415,and thus compensating the local shift of the horizontal lines 1415. Thedefect compensation will bring the lines 1415 in close agreement withthe lines 1310 of FIG. 13, whereby the reference pattern elements 1455are shifted to the positions of the reference pattern elements 1350. Aglobal shift of all feature elements of the first mask can becompensated by a scaling and/or orthogonality correction of the scannerof the photolithographic exposure system.

FIG. 15 schematically represents a cut-out 1500 of the 2D pattern 1300of FIG. 13, wherein the line width of the vertical lines 1535 increasesfrom right to left. This is accompanied by a respective decrease ofspaces 1545 between the vertical lines 1535. The inhomogeneous linewidth 1535 is caused by a critical dimension uniformity (CDU) variationof the pattern of the second mask used for the generation of thevertical lines 1535. As shown in FIG. 15, the reference pattern elements1350 of FIG. 13 are deformed from cycles to ellipses 1555 and 1560,whereby their vertical half axes represents the varying space 1545between the vertical lines 1535. It has here to be stressed that theelliptical deformation is just an example and that any deviation of a 2Dor 3D structure from its original form can be used to detect a CDUvariation.

The ellipses 1555 and 1560 schematically represent the impact of a localCDU variation on the finally printed 2D pattern distortion. The majorand minor axes of the ellipses 1555 and 1560 can for example be measuredwith a SEM and this metric can be used to calculate the overlay error.

The overlay problem of FIG. 15 can be compensated by writing at leastone arrangement of pixels in the substrate of the second mask whichintroduces a variation of the optical transmission in the second mask asdiscussed above in the context of FIG. 12.

The pattern cut-out 1600 of FIG. 16 shows the 2D pattern 1300 of FIG. 13having a local shift in the horizontal lines 1615 of FIG. 14 relative tothe ideal lines 1310 of FIG. 13. Furthermore, the vertical lines 1625have the CDU error of FIG. 15. The shift and deformation of thereference pattern elements 1350 of FIG. 13 are indicated in FIG. 16 bythe shifted ellipses 1655 and 1660. FIG. 17 depicts the enlargedellipses 1655 and 1660 of FIG. 16.

The errors of the two masks are corrected as explained during thediscussion of FIGS. 14 and 15. The shift as well as the dimensions ofthe major and the minor axes of the ellipses 1655 and 1660 can again bemeasured with a high-resolution metrology tool, as for example a SEM.Similar to FIG. 14, the local shift of the vertical lines 1615 can causeplacement errors of the reference pattern elements 1350 which are forexample contact holes (CHs). Additionally, it may lead to somedistortion issues. The critical dimension (CD) non-uniformity will causea CD distortion. Again analog to FIG. 15, the CDU error of the secondmask used for the generation of the vertical pattern 1635 typicallyresults in a distortion of the reference pattern elements 1350 asschematically indicated by the ellipses 1655 and 1660.

FIG. 18, which is taken from the paper “Towards 3 nm overlay andcritical dimension uniformity: an integrated error budget for doublepatterning lithography” of W. A. Arnold mentioned in the second part,illustrates schematically some important process steps of a self-aligneddouble patterning (SADP) process. A layer 1820 to be etched or patternedis arranged on a wafer 1810. On top of the layer 1820 a so-called hardmaterial photo resist of a sacrificial layer 1830 is deposited. Thesacrificial layer 1830 is a photo resist or a hard mask layer and maycomprise silicon nitride, silicon oxide, or any other appropriatematerial for this purpose or function. A photo resist layer 1840 isarranged on the sacrificial layer 1830. The wafer 1810 is then exposedto a photolithographic mask which forms dense pattern elements 1850 andisolated pattern elements 1870. In the example of FIG. 18, the patternelements 1850 and 1870 have a nominal width of 1855 or 1875,respectively. The actual widths 1860 and 1880 of the pattern elements1850 and 1870 on the wafer 1810 are smaller than the nominal orpredetermined width 1855 or 1875, respectively. The small widths 1860and 1880 generated in the lithography step cause a CDU error in thelayer 1820 to be etched.

Further, the spacer deposition and the subsequent etching process (asindicated in FIG. 11) introduce a further variation in the spacerelements 1890 formed by the etching process. After the removal of theremaining photo resist 1840 and of the sacrificial layer 1830, thepattern formed by the spacer elements 1890 is etched in the layer 1820,wherein the spacer elements 1890 act as etch stop elements. Finally, thespacer elements 1890 and the sacrificial layer portions below the spacerelements 1890 are removed from the generated pattern elements of thelayer 1820.

In the etching process a variation of the lithography pattern 1850, 1870or a CD error transforms in an error of the widths of the stripes S2 ascan be seen in the lower part of FIG. 18. Furthermore, the etchingprocess converts a variation of the spacer elements 1890 in a variationof the lines L1 and L2. A variation of the lines L1 and L2 also resultsin a variation of the spaces S1. Consequently, the pattern generatedwith the SADP process shows an overlay error; an effect which is knownas “pitch walking”.

The errors in the one-dimensional (1D) line space pattern of the SADPprocess can again be measured by using the metrology tool described inFIG. 3, by using for example a SEM and/or a scatterometer.

The defined method enables to correct the defects of the SADP pattern ofFIG. 18. In a first step at least one arrangement of pixels isintroduced or written in the photolithographic mask used for thegeneration of the pattern 1850 and 1870 in a lithography process. Thearrangement(s) of pixels compensate the CDU variation by introducing arespective variation of the optical transmission in the substrate of thephotolithographic mask. This process brings the spaces S2 in FIG. 18 inclose agreement with the predetermined space.

In a second step, an arrangement of local persistent modifications isintroduced in the sacrificial layer 1830. The local persistentmodifications or the pixels in the sacrificial layer 1830 induce astructural change of the material of the sacrificial layer 1830 and thusavoiding a variation of the lines during the etching of the layer 1820.The modified sacrificial layer 1830 prevents “pitch walking” effectsduring the etching step of the layer 1820. As a consequence the lines L1and L2 are generated having a uniform width. The pixels are written inthe sacrificial layer 1830 prior to the deposition of the photo resist1840. In case the sacrificial layer 1830 is placed wrong, it is alsopossible to apply a RegC process in order to correct the local as wellas the wrong placement of the sacrificial layer 1830, while the scannercan correct only global (low frequency) overlay errors.

This means that a critical dimension non-uniformity (CDNU) in asacrificial layer 1830 leads to overlay error in a SADP process. Thiserror can be measured by conventional wafer fab metrology tools orsystems such as but not limited to: CD SEM (critical dimension scanningelectron microscope), AFM, scatterometry and/or profilometry. Then, theidentified overlay error can be fixed by applying at least onearrangement of local persistent modifications in the sacrificial layer1830 similar than described above for the substrate of aphotolithographic mask.

In the following, a further example for the application of the inventiveprinciple is presented. In this example, the at least one error on awafer is measured with the aid of a test mask. For this purpose, a testmask is designed and created which has test features which are alignedwith at least one mask used for the fabrication of ICs on a wafer. Thismask is in the following also called a production mask. The testfeatures are in-die test features in order to secure a high resolutionat the error detection.

FIG. 19 schematically presents a test mask 1900 having a test pattern1910. The actual test pattern 1910 of the test mask 1900 is designed byIC or device designers. It is also beneficial to consult opticalproximity correction (OPC) designers at the determination of the layoutof the test pattern 1910 of the test mask 1900. The test pattern 1910 ofFIG. 19 comprises dense pattern elements 1920, semi dense (or “denso”)pattern elements 1930, and isolated pattern elements 1940.

The test mask 1900 is investigated in detail on mask level. Further, thepattern printed by the test mask 1900 on a wafer is also investigatedwith respect to CDU and pattern placement errors. Thus, the test mask1900 can be regarded as a reference test mask which prints its patternelements 1910 in a minimal and yet well mapped and documented CDU andpattern placement error.

The test mask 1900 is presently preferably applied in combination with asecond mask or a production mask in a double patterning process. It ishowever appreciated that the test mask concept can also be applied inthe fabrication process of a wafer using a single exposure lithographytechnology. In the following, the concept is discussed in the context ofa DPL process.

In a first step, the test mask 1900 is exposed to a wafer. After thedevelopment of the exposed photo resist, the test pattern 1910 of thetest mask 1900 is etched in the wafer. The combination of the exposureand etching process is also called printing of the test mask 1900. Inthe second step, the photolithographic mask used in the fabrication of aDPL layer on the wafer or a production mask is also printed on the waferas described above for the test mask 1900.

Standard very large scale integration (VLSI) processes require a surfaceplanarization in order to avoid a problem with the depth of focus (DOF)due to a non-planar surface topography of the photo resist on the wafer.The proposed method overcomes this challenge. In order to minimizedeviations from a standard fabrication process, it is howeveradvantageous to print the test mask 1900 on a thin layer with a highoptical contrast to ease the metrology process, as this layer can betreated like a hard mask, for example a silicon nitride layer, and toperform a full lithography and etch process on the test mask followed bya printing process of the production mask used in the DPL process andthen performing the overlay and CD measurements in-die on the wafer.

FIG. 20A schematically shows a test pattern 2010 arranged on the nitridelayer 2020 of the wafer 2000. The test pattern 2010 has been generatedin a first litho-etch (LE) process (right part of FIG. 20A). Afterperforming an etching step and after applying a second photo resist2030, the hard mask layer 2020 allows a minimal topography, i.e. isessentially flat. This is indicated in the left part of FIG. 20A. On theother hand, FIG. 20B illustrates an example wherein the wafer 2005 doesnot have a hard mask layer 2020, and the surface of the photo resist2035 shows a distinct topography 2045. The topology can negativelyaffect both, the photolithography process and the metrology resolution,accuracy and precision. Therefore, it is recommended to us the method ofFIG. 20A.

In the next step, the overlay error between the test mask 1900 and theproduction mask is measured. This can be performed by using themetrology tool described in FIG. 3, by using a SEM, and/or by using ascatterometer and/or by using additional image base or modeling basemetrology methods. The overlay error map of the overall wafer isdetermined by stacking the relevant sampling fields of the individualexposures of the test mask 1900 and the production mask across thewafer.

The overlay errors on wafer level are then compensated by writing one ormore arrangements of pixels in the substrate of the production mask. Theprocess of printing and measuring of the test mask 1900 and thecorrected production mask is then repeated in order to check whether thecorrection process by the writing of pixel arrangements has beensuccessful.

The described procedure is performed for all overlay critical productionmasks in order to minimize the overall overlay error budget. It is inthis process beneficial to include in the test pattern 1910 of the testmask 1900 pattern elements that allow checking of the patterns of asmany as possible production masks. This procedure minimizes the cost ofthe test mask concept as only one or a very limited number of test masks1900 are required. On the other hand, this procedure also reduces errorintroduced at the transition from a first test mask to a second testmask. This is shortly explaining with the aid of the following examples.

The first example describes the application of a single test mask 1900for the control of several patterns of several layers of a wafer. Twolayers, called A and B, are supposed to the overlay critical layers. Thepatterns P of the layers A and B at the position i are called PA_(i) andPB_(i). A test pattern 1910 of a test mask 1900 has test pattern TA_(i),and TB_(i), wherein the test pattern TA_(i) and TB_(i) are close to eachother. This allows the assumption that the placement error of the testpattern TA_(i) and TB_(i) are identical. This means that one testpattern T_(i) can actually serve for the test of both pattern PA_(i) andPB_(i):

TA _(i) =TB _(i) =T _(i)  (1)

In a first step, the test mask 1900 and the mask or production mask forthe layer A are printed by using a LELE or a LFLE process and thedisplacements of the pattern elements relative to the test pattern aremeasured:

ΔA _(i) =PA _(i) −TA _(i) =PA _(i) −T _(i)  (2)

Then, the test mask 1900 and the production mask for the layer B of thewafer are printed in a LELE of LFLE process, and the resultingdisplacements between the test pattern 1910 and the pattern of mask Bare determined:

ΔB _(i) =PB _(i) −TB _(i) =PB _(i) −T _(i)  (3)

From the two measurements presented in equations 2 and 3 the relativedisplacements of the pattern elements PA_(i) and PB_(i) can bedetermined:

ΔAB _(i) =PA _(i) −PB _(i) =ΔA _(i) −T _(i)−(ΔB _(i) −T _(i))=ΔA _(i)−ΔB _(i)  (4)

As already mentioned, the relative location of the test pattern TA_(i)and TB_(i) can additionally be obtained from a detailed analysis of thetest mask 1900 using one or more of the metrology tools described above.

In the following example, two test masks A and B are designed, whereinthe test mask A comprises the test pattern TA_(i) and the second testmask B comprises the test pattern TB_(i). The test pattern of each testmask contains an alignment element AL, as for example an Archeralignment mask. It is again assumed that the test patterns TA_(i) andTB_(i) are close to the alignment AL, so that the placement error of thetest patterns TA_(i) and TB_(i) relative to the respective alignmentelement AL can be neglected.

The test masks A and B are applied in order to check two overlaycritical layers A and B of a wafer. For this purpose, a test print ofthe mask A is performed and the locations of the test pattern TA_(i) aredetermined by using the alignment marks ALA_(i):

TA _(i) =ALA _(i) −ΔTA _(i)  (5)

Then, a test print of the mask B is performed and the locations of thetest pattern TB_(i) are determined by using the alignment marks ALB_(i):

TB _(i) =ALB _(i) −ΔTB _(i)  (6)

In a first LELE or LFLE process the test pattern TA_(i) of the test maskA and the pattern PA_(i) of the production mask A are printed. Thisresults in displacements of the pattern elements PA_(i) relative to thetest pattern TA_(i):

ΔA _(i) =PA _(i) −TA _(i) =PA _(i)−(ALA _(i) +ΔTA _(i))=PA _(i) −ALA_(i) −ΔTA _(i)  (7)

In a second print process the test pattern TB_(i) of the test mask andthe pattern P_(i) of the production mask B are printed which leads todisplacements of the pattern elements PB_(i) relative to the testpattern TB_(i) of test mask B:

ΔB _(i) =PB _(i) −TB _(i) =PB _(i)−(ALB _(i) +ΔTB _(i))=PB _(i) −ALB_(i) −ΔTB _(i)  (8)

From the two measurements the relative displacements PA_(i) and PB_(i)are known according to:

ΔAB _(i) =PA _(i) −PB _(i) =ΔA _(i)+(ALA _(i) +ΔTA _(i))−(ΔB _(i)+(ALA_(i) +ΔTA _(i)))  (9)

wherein the terms in the brackets are determined from the test prints ofthe test masks A and B as described above.

A possible variation of the presented procedure is the design of thetest pattern TA_(i) and TB_(i) that allow a measurement of theirrelative position. In this case, the alignment marks AL are redundantand can serve for further qualification purposes.

FIG. 21 schematically illustrates a field of a wafer layout after thetest pattern 1910 of the test mask 1900 of FIG. 19 (dark grey elements)and a vertical pattern 2110 (light grey elements) of a production maskare printed in a LELE of an LFLE process. The production mask comprisesdense vertical lines 2120, medium dense lines 2130 and one isolated line2140. For the production mask of FIG. 21 only vertical lines of the testpattern 1910 of the test mask 1900 are relevant. The dense lines 2120and the isolated line 2140 of the production mask pattern and of thetest mask pattern 1910 are essentially aligned to each other, whereasthe medium dense line 2130 has a local line space error 2150 withrespect to the respective test pattern element of the test mask 1900.The detected local overlay error 2150 between the test pattern elementand the vertical line pattern of FIG. 21 is caused by an in-dieplacement or registration error. It can be corrected by writing anarrangement of pixels in the range of the medium dense lines 2130 of themask of FIG. 21 in order to compensate the defect of the production maskof FIG. 21.

FIG. 22 schematically presents the situation in which the test maskpattern 1910 of the test mask 1900 is printed in a litho-etch (LE)process. In a first step, a LE process of the LELE process the contactholes of a production mask are printed and etched. The contact holes(CHs) are indicated in FIG. 22 by cycles. Many thousands CHs are etchedin a field of a layer of a wafer which forms a layer of a modern IC.Only five of them are important for a test of the CH arrangement by thetest pattern 1910 of the test mask 1900. These are depicted in FIG. 22by dark grey circles 2220, 2230, 2240, 2250, and 2260. Four smallsquares 1915 of the test pattern 1910 are used to check the placement ofthe CH arrangement.

As can be seen from FIG. 22, the four test CHs 2220, 2230, 2240, and2250 are essentially at their predetermined position, whereas the CH2260 is shifted with respect to its predetermined location. Theplacement error of the contact hole 2260 can be corrected by writing anarrangement of pixels in the range of the erroneous CH 2260, whichshifts the CH 2260 essentially to its predetermined position. The in-dieerror determination provides an indication where to position the atleast one arrangement of local persistent modifications used for thedefect compensation. The expression “essentially” means here as well ason other positions within this description a positioning within theresolution limit of a state-of-the-art metrology tool.

Similar to FIGS. 21 and 22, FIG. 23 depicts a wafer pattern printed andetched with the test pattern 1910 of the test mask 1900 of FIG. 19 and aproduction mask having a horizontal line space pattern. For this usecase, only the horizontal bars 1925, 1935 and 1945 of the test pattern1910 are relevant and used for a check of the horizontal lines 2320 and2330 of the production mask of FIG. 23. As can be seen from FIG. 23, thelines 2320 and 2330 of the production mask are simply vertically shiftedrelative to the test pattern elements 1925, 1935 and 1945. This meansthat the detected error is a global overlay error. As already explainedat the discussion of FIG. 19, the test pattern 1910 of the test mask1900 is investigated in great detail, so that the test pattern can betaken as a reference pattern. Thus, the shift detected between the testpattern elements 1925 and 1935 and the line 2320 and the test patternelement 1945 and the line 2330 can be attributed to a shift of the lines2320 and 2330 of the production mask. Therefore, the test mask pattern1910 can not only be used to detect local errors of a production mask,but can also be used analyze global overlay errors of a production mask.

The displacement error of the horizontal lines 2320 and 2330 of the maskof FIG. 23 detected by in-die measurements is compensated by a lineartransformation (scaling and orthogonality correction) of the scannerused in the DPL process. This means that the test mask concept, which isbased on in-die measurements, can also be used to correct global overlayerrors, and thus minimizing the overlay error of overlay error tight DPLprocesses as well errors of single patterning processes where theoverlay is measured between layers in different topographic planes on awafer.

Alternatively, it is also possible to write arrangements of pixels inthe mask of FIG. 23 in order to bring lines 2320 and 2330 in line withthe test pattern elements 1925, 1935 and 1945.

FIG. 24 schematically presents a superposition of a field of a waferlayout of the three various layers of FIGS. 21 to 23. The correctiveactions of the individual masks in order to minimize the overlay errorof the various masks have been indicated at the discussion of thevarious layers. FIG. 25 schematically represents the superposition ofthe various layers of FIG. 24 after the errors of the individual errorshave been corrected with respect to the corresponding pattern elementsof the test pattern 1910 of the test mask 1900. As can be seen from FIG.25, the overlay error of the layers of FIGS. 21 to 23 is minimized bythe corrective actions of each mask.

The examples presented up to now refer to a single exposure of a mask ona wafer. However, a wafer layout comprises the subsequent exposure of amask many times in order to cover the overall wafer area with thepattern of a mask in order to print a layer of an IC to be fabricatedmany times on the wafer. A single exposure of a mask in on a wafer iscalled a field. Thus, the error discussed up to now are in-field errors.

In the following, a data analysis for an overlay error determination andcompensation on the wafer level is discussed. To be more precise, amethod for the determination of a route mean square (RMS) overlay erroracross a wafer and its compensation is presented. FIG. 26 schematicallyrepresents a field 2600 of a single exposure or a multiple exposure asfor example a double exposure of a mask. The field comprises threein-field metrology sites 2610, 2620 and 2630 at which the in-fieldoverlay error is determined. It is important to emphasize that the abovemethods allow measurements of an in-die overlay metrology on realproduct features, and thus providing a unique opportunity to control theoverlay of product features rather than test features that may or maynot be correlative to the actual device features' overlay.

FIG. 27 schematically shows a wafer 2700 which is covered by fields 2720of subsequent exposures of a photolithographic mask. In a LELE or LFLEprocess two masks are required for a single in-field exposure. Each ofthe fields 2600, 2720 on the wafer 2700 has the overlay in-field targets2610, 2620 and 2630 of FIG. 26. As is indicated by the crossed overlaytargets in FIG. 27, an area from a sub-millimeter range up to severalmillimeters from the edge 2710 of the wafer is excluded from thedetermination of the overlay error across the wafer 2700. This measureexcludes wafer edge bias effects at the determination of the overlayerror at wafer level. Based on wafer fab experience, the edged dieeffect cannot be compensated by lithography processes and shouldtherefore not taken into account while averaging metrology data of allfields as an input of RegC and CDC processes.

In the next step, the overlay error on field level measured at multiplepoints per field, three positions 2610, 2620 and 2630 are shown in FIG.26. For the determination of the in-field overlay error one of themethods presented above can be used. The in-field measurement focuses onhot spots in the exposure area of a mask in order to have sufficientexperimental data for a statistically significant in-field errordetermination. A full wafer or a field sampling can be used to create anaverage field-level error based on averaging of each of the targets atthe sampled fields.

The in-field overlay error map of the individual fields 2600, 2720 arethen stacked. The mask is then corrected by writing at least onearrangement of pixels in the substrate of the mask. A new wafer isprinted with the corrected mask based on average metrology of all waferfields except for edge-affected fields and the correction process isapplied on this average in order to achieve a minimal error across thewafer. The overlay error resulting from the corrected mask is measuredon both, the in-field level and the wafer level in order to check theerror compensation improvement.

It is important to note that the number of measurement points per fieldshould be defined by the manufacturer of the device fabricated from theprocess wafer in order to optimize the trade-off between the correctionefficiency and the metrology productivity as more measurement sitesmeans more metrology time and higher cost of the end-product. Thediscussed methods are relevant to work with any given sampling plan ofthe device manufacturer.

FIG. 28, which is taken from an internet publication, presents anexample of an overlay error map 2810 across a wafer 2800. In order toobtain the error contributions which cannot be compensated by a scalingor orthogonality (S/O) correction, these errors are removed from theoverlay error map 2810 of FIG. 28. The resulting overlay error map 2900is presented in FIG. 29. In the in-field error map 2900 of FIG. 29 threeregions 2910, 2920 and 2930 can be identified as examples of areas inthe field which have a systematic error.

FIG. 30 illustrates the remaining in-field error distribution 3010 ofFIG. 29 after the systematic errors 2910, 2920 and 2930 have beencorrected by introducing respective arrangements of local persistentmodifications in the substrate of the photolithographic mask. Thesystematic errors 2910, 2920, and 2930 of the combined mask andlithography process, which result in an increase of the respectiveoverlay error, are essentially removed.

1. (canceled)
 2. A method for correcting at least one error on wafersprocessed by at least one photolithographic mask and/or a template for ananoimprint lithography, the method comprising: a. measuring the atleast one error on a wafer at a wafer processing site; and b. modifyingthe at least one photolithographic mask and/or the template for thenanoimprint lithography by introducing at least one arrangement of localpersistent modifications in the at least one photolithographic maskand/or the template for the nanoimprint lithography.
 3. The methodaccording to claim 2, wherein the at least one error on the wafercomprises at least one of: a pattern placement error, at least onecritical dimension uniformity error, at least one overlay error of aplurality of photolithographic masks and/or templates for thenanoimprint lithography, at least one pellicle mounting error, or atleast one imaging error of a photolithographic projection exposuresystem.
 4. The method according to claim 3, wherein the at least oneoverlay error comprises at least one error of at least one of at leasttwo photolithographic masks used in a multiple patterning lithographyprocess.
 5. The method according to claim 2, wherein measuring of the atleast one error comprises measuring of the at least one error in theactive area of a chip (in-die).
 6. The method according to claim 5,wherein in-die measuring of the at least one error comprises measuringof at least one of: a shift of at least one two-dimensional structure ona wafer, a shift of at least one three-dimensional structure on a wafer,an ellipticity of at least one two-dimensional structure with animaging-based or a model-based metrology method, or an ellipticity of atleast one three-dimensional structure with an imaging based or amodel-based metrology method.
 7. The method according to claim 3,wherein the error comprises at least one overlay error of at least onefirst photolithographic mask and at least one second photolithographicmask and/or of the at least one first template for the nanoimprintlithography and the at least one second template for the nanoimprintlithography, the method further comprising the step of: correcting theat least one overlay error by introducing at least one arrangement oflocal persistent modifications in the at least one firstphotolithographic mask and/or in the at least second photolithographicmask and/or in the at least one first and/or the at least one secondtemplate so that the at least one overlay error is minimized.
 8. Themethod according to claim 7, wherein correcting the at least one overlayerror comprises introducing at least one first arrangement of localpersistent modifications in the at least one first photolithographicmask and/or introducing at least one second arrangement of localpersistent modifications in the at least one second photolithographicmask, and/or introducing at least one first arrangement of localpersistent modifications in the at least one first template and/orintroducing at least one second arrangement of local persistentmodifications in the at least one second template.
 9. The methodaccording to claim 7, wherein the at least one overlay error comprisesat least one critical dimension uniformity error of the firstphotolithographic mask and/or at least one pattern placement error ofthe second photolithographic mask in a multiple patterning lithographyprocess.
 10. The method according to claim 2, wherein measuring the atleast one error on the wafer comprises: a. generating a test mask havinga test pattern; b. printing and etching the test pattern of the testmask on the wafer; c. printing and etching a photolithographic maskpattern or a template pattern on the test pattern of the wafer; and d.determining the at least one error as a difference of at least onepattern element of the photolithographic mask or the template for thenanoimprint lithography and of the at least one respective test patternelement of the test mask.
 11. The method according to claim 10, whereinprinting and etching of the test mask and printing and etching of thephotolithographic mask comprises a single patterning or a multiplepatterning lithography process.
 12. The method according to claim 10,further comprising the step of determining an overlay error from a shiftof a plurality of test pattern elements of the test mask relative topattern elements of the photolithographic mask used in the single ormultiple patterning lithography process, or determining an overlay errorfrom a shift of a plurality of test pattern elements of the test maskrelative to pattern elements of the template.
 13. The method accordingto claim 2, further comprising the step of introducing the at least onearrangement of local persistent modifications in a sacrificial layer onthe wafer used in a self-aligned double patterning process.
 14. Anapparatus for correcting at least one error on wafers processed by atleast one photolithographic mask and/or a template for a nanoimprintlithography, comprising: a. at least one metrology system located in awafer processing site and/or in a mask shop and adapted to measure theat least one error on a wafer; b. at least one computing means adaptedto calculate parameters for at least one error correcting means based onthe at least one error; and c. the at least one error correcting meansadapted to introduce at least one arrangement of local persistentmodifications in the photolithographic mask and/or in the template forthe nanoimprint lithography by applying ultra-short light pulses. 15.The apparatus according to claim 14, wherein the apparatus is adapted tocorrect at least one error on wafers processed by at least onephotolithographic mask and/or one template for the nanoimprintlithography by: a. measuring the at least one error on a wafer at awafer processing site; and b. modifying the at least onephotolithographic mask and/or the template for the nanoimprintlithography by introducing at least one arrangement of local persistentmodifications in the at least one photolithographic mask and/or thetemplate for the nanoimprint lithography.
 16. The apparatus according toclaim 14, wherein the at least one metrology system comprises anultrahigh-precision stage, at least one laser source or at least oneother light source and at least one charge-coupled device cameraoperating in the ultraviolet wavelength range, a scanning electronmicroscope or a scatterometer, and an image-based or a model-basedmetrology system.